Don't use a hidden value of certname if use_uuid_for_certificates has been toggled on/off

Expected:
* Enable use_uuid_for_certificates
* Create a host
* Host has a UUID now
* Set host to Build, pull the host's template, watch the certname UUID value be added to the Puppet Autosign file.
* ... realize that uuid certificates aren't right for your organization ...
* Disable use_uuid_for_certificates
* Set host to Build, pull the host's template, watch the _certname hostname value_ be added to the autosign file.

Actual:
* Set host to Build, pull the host's template, watch the _certname UUID value_ be added to the Puppet Autosign file.
* Watch as your Puppet run fails to get a certificate because the client has submitted a certificate request using its hostname now.

Patch fixes this by checking for state of the use_uuid_for_certificates setting in the certname model method.